1. Field of the Invention
The present invention generally relates to a magnetic resonance (MR) imaging method and an MR imaging system. More specifically, the present invention is directed to such MR imaging method/system capable of compensating for a disturbed gradient field caused by an eddy current field.
2. Description of the Prior Art
A magnetic resonance (MR) imaging system has been widely utilized in the medical electronic field. In an MR imaging system, a static field is produced from a main magnet wound on an air-core. A biological body under medical examination such as patient is laid within this air-core of the main magnet under influence of the static field. When an RF pulse is applied to this patient under the above-described condition, an MR signal is induced therein. Normally, to acquire positional information of the MR signal, three-orthogonal gradient fields are applied to the patient, the strengths of which are gradually changed along the three orthogonal axes. The three-orthogonal gradient fields are produced by the gradient coils driven by the gradient field power source.
When the drive currents are supplied from the gradient field power source to produce the gradient fields, an eddy current happens to occur in an electrically conductive member which constitutes the main magnet. Accordingly, the three-orthogonal gradient fields are disturbed, or deformed from the desirable gradient fields due to the eddy current. In this case, the image quality of the MR image is deteriorated.
To avoid such an image deterioration, the following conventional eddy current compensating methods have been proposed. That is, the active gradient coil system has been employed as the gradient field coil system, in which the gradient fields are generated only in the MR imaging region and also give no influence to the main magnet field. Alternatively, the eddy current compensating currents are superimposed with the drive current to the gradient coils.
However, the conventional active gradient coil system has a practical drawback in use that very high precision is required in manufacturing the complex coil construction, and total cost of this gradient coil system becomes very expensive.
In case of the eddy current compensating method, if both of the gradient field coil and the portion of the main magnet where the eddy current is produced are completely fixed and also no such an LRC circuit magnetically coupled to the gradient field within the main magnet is present, disturbance in the gradient field caused by the eddy current may be avoided. However, such an LRC circuit is actually present within the main magnet, and/or the portion of the main magnet where the eddy current is induced is mechanically vibrated. As a result, a magnetic field having a damped vibrating waveform is produced, which may disturb the gradient fields. Therefore, the image quality of the MR image is deteriorated. Furthermore, if the objects for generating the eddy current fields are arranged in an asymmetrical form with respect to the gradient field coils, even-numbered components of the eddy current fields may cause the following problems. These objects are made of, for instance, a metal cylinder for a thermal-shielding purpose in the main magnet. For example, as described in Japanese KOKAI (Disclosure) Patent Application No. 63-82638 (1988), when the metal cylinders are positioned in an asymmetrical shape with respect to the gradient field coils, the even-numbered components of the eddy current fields are produced along the vertical direction, which may disturb the gradient fields. To connect such a disturbance of the gradient fields, the correction coil is employed.
Also, as described in Japanese Patent Application No. 2-144040 (1990), the entire gradient field coil system is subdivided into two coil systems, and the correction gains of these subdivided coil systems are independently determined. However, since the gradient field coil system should be subdivided into two coil systems, two sets of large-scaled constant current sources for producing the major magnetic fields are required, resulting in higher installation cost. Furthermore, there is another problem that the eddy current fields with different numbers along the vertical direction cannot be corrected.